In color diffusion transfer photography, a means of stabilizing an image that is formed by spreading an alkaline processing solution after exposure is known, in which the pH value of the image-forming system is lowered to about the neutral point (pH=7) after the image formation. In particular, this operation is important in so-called monosheet type color diffusion transfer photography, in which if the image formed is stored in a high pH system, the optical density of the highlight part would increase due to heat or moisture and the image quality would often be noticeably deteriorated thereby. This is because the non-reacted dye precursor (which is an alkaline compound capable of releasing a dye by redox reaction with the oxidation product of a developing agent) still existing in the light-sensitive layer is gradually oxidized by oxygen in the air to thereby release a dye. The reaction may proceed more easily in a higher pH system, and therefore, it is effective to lower the pH value in the system as much as possiblde so as to overcome such trouble. However, if the pH value of the system is too low, the dye as mordanted in the mordant layer is decomposed by light or oxygen and the image formed can even be lost in an extreme case. Accordingly, the improvement of image storability in a dark or light place under high temperature and high humidity could not be attained by only the pH adjustment of the photographic system during storage and any other means is being required therefor.